David L. Walker

Phasic vs Sustained Fear in Rats and Humans: Role of the Extended Amygdala in Fear vs Anxiety

Data will be reviewed using the acoustic startle reflex in rats and humans based on our attempts to operationally define fear vs anxiety. Although the symptoms of fear and anxiety are very similar, they also differ. Fear is a generally adaptive state of apprehension that begins rapidly and dissipates quickly once the threat is removed (phasic fear). Anxiety is elicited by less specific and less predictable threats, or by those that are physically or psychologically more distant. Thus, anxiety is a more long-lasting state of apprehension (sustained fear). Rodent studies suggest that phasic fear is mediated by the amygdala, which sends outputs to the hypothalamus and brainstem to produce symptoms of fear. Sustained fear is also mediated by the amygdala, which releases corticotropin-releasing factor, a stress hormone that acts on receptors in the bed nucleus of the stria terminalis (BNST), a part of the so-called ‘extended amygdala.’ The amygdala and BNST send outputs to the same hypothalamic and brainstem targets to produce phasic and sustained fear, respectively. In rats, sustained fear is more sensitive to anxiolytic drugs. In humans, symptoms of clinical anxiety are better detected in sustained rather than phasic fear paradigms.

Role of the bed nucleus of the stria terminalis versus the amygdala in fear, stress, and anxiety

The bed nucleus of the stria terminalis is a limbic forebrain structure that receives heavy projections from, among other areas, the basolateral amygdala, and projects in turn to hypothalamic and brainstem target areas that mediate many of the autonomic and behavioral responses to aversive or threatening stimuli. Despite its strategic anatomical position, initial attempts to implicate the bed nucleus of the stria terminalis in conditioned fear were largely unsuccessful. Recent studies have shown, however, that the bed nucleus of the stria terminalis does participate in certain types of anxiety and stress responses. In this work, we review these findings and suggest from the emerging pattern of evidence that, although the bed nucleus of the stria terminalis may not be necessary for rapid-onset, short-duration behaviors which occur in response to specific threats, the bed nucleus of the stria terminalis may mediate slower-onset, longer-lasting responses that frequently accompany sustained threats, and that may persist even after threat termination.

Double Dissociation between the Involvement of the Bed Nucleus of the Stria Terminalis and the Central Nucleus of the Amygdala in Startle Increases Produced by Conditioned versus Unconditioned Fear

The amplitude of the acoustic startle response is reliably enhanced when elicited in the presence of bright light (light-enhanced startle) or in the presence of cues previously paired with shock (fear-potentiated startle). Light-enhanced startle appears to reflect an unconditioned response to an anxiogenic stimulus, whereas fear-potentiated startle reflects a conditioned response to a fear-eliciting stimulus. We examine the involvement of the basolateral nucleus of the amygdala, the central nucleus of the amygdala, and the bed nucleus of the stria terminalis in both phenomena. Immediately before light-enhanced or fear-potentiated startle testing, rats received intracranial infusions of the AMPA receptor antagonist 2,3-dihydroxy-6-nitro-7-sulphamoylbenzo(F)-quinoxaline (3 μg) or PBS. Infusions into the central nucleus of the amygdala blocked fear-potentiated but not light-enhanced startle, and infusions into the bed nucleus of the stria terminalis blocked light-enhanced but not fear-potentiated startle. Infusions into the basolateral amygdala disrupted both phenomena. These findings indicate that the neuroanatomical substrates of fear-potentiated and light-enhanced startle, and perhaps more generally of conditioned and unconditioned fear, may be anatomically dissociated.

The role of amygdala glutamate receptors in fear learning, fear-potentiated startle, and extinction.

Using a paradigm known as fear-potentiated startle, we have examined the neurobiological substrates of Pavlovian fear conditioning. In these experiments, rats are trained to fear an initially neutral stimulus by pairing that stimulus with shock. The amount of fear elicited by the stimulus [i.e., now a conditioned stimulus (CS)] is later assessed by presenting startle-eliciting noise bursts both in the presence and also the absence of the CS. After training, startle responses are typically greater in the presence of the CS. Findings reviewed here suggest that amygdala N-methyl-D-aspartate (NMDA) receptors play a key role in triggering the neural changes that support fear learning and also the loss of fear that accompanies extinction training. Amygdala (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors also participate in fear learning. However, unlike NMDA receptor antagonists, AMPA receptor antagonists also block fear-potentiated startle when infused prior to testing. Very recent data indicate that glutamate metabotropic Group II receptor agonists also block fear learning when infused into the amygdala prior to training, and block fear-potentiated startle when infused prior to testing. A fuller understanding of the role of amygdala glutamate systems in fear and fear learning may suggest novel pharmacological approaches to the treatment of clinical anxiety disorders.

Roles of the Amygdala and Bed Nucleus of the Stria Terminalis in Fear and Anxiety Measured with the Acoustic Startle Reflex

Over the last several years, our laboratory has been studying how a simple reflex, the acoustic startle reflex, can be modified by prior emotional learning. Thus far, most of our work has concentrated on an experimental paradigm called the fearpotentiated startle effect, in which the amplitude of the startle reflex can be modified by a state of fear. More recently, however, we are trying to develop experimental methods to measure both fear and anxiety using changes in the acoustic startle reflex. Fear is a natural, adaptive change in an organism elicited by a potentially threatening stimulus which prepares the organism to cope with the provocation. Fear generally is elicited by a clearly identifiable stimulus and subsides shortly after its offset. Anxiety also is a change in the state of an organism which has many of the same signs and symptoms of fear. However, it may not be clearly associated with a single eliciting stimulus, may last a long time once activated, and may lack clear adaptive significance.

Anxiogenic effects of high illumination levels assessed with the acoustic startle response in rats

Previous studies have shown that the amplitude of the acoustic startle reflex is increased by the presentation of aversive stimuli. In the present study, the amplitude of acoustic startle in rats was increased by exposure to high illumination levels. The effect was directly related to the intensity (0, 8, 70, and 700 footlamberts) of illumination (experiment I); was blocked by the anxiolytic compound buspirone (experiment II); and showed little or no habituation with repeated testing (experiment III). These results suggest that the elevation of startle amplitude by light may reflect an unconditioned anxiogenic effect of high illumination levels. The possible utility of this phenomenon as an animal model of anxiety is discussed.

Role of the extended amygdala in short-duration versus sustained fear: a tribute to Dr. Lennart Heimer

The concept of the “extended amygdala”, developed and explored by Lennart Heimer, Jose de Olmos, George Alheid, and their collaborators, has had an enormous impact on the field of neuroscience and on our own work. Measuring fear-potentiated startle test using conditioned stimuli that vary in length we suggest that the central nucleus of the amygdala (CeA) and the lateral division of the bed nucleus of the stria terminalis (BNSTL) are involved in short-term versus long-term fear responses we call phasic versus sustained fear, respectively. Outputs from the basolateral amygdala (BLA) activate the medial division of the CeA (CeAM) to very rapidly elicit phasic fear responses via CeAM projections to the hypothalamus and brainstem. The BLA also projects to the BNSTL, which together with other BNSTL inputs from the lateral CeA (CeAL) initiate a slower developing, but sustained fear response, akin to anxiety. We hypothesize this occurs because the CeAL releases the peptide corticotropin releasing hormone (CRF) into the BNSTL which facilitates the release of glutamate from BLA terminals. This activates the BNSTL which projects to hypothalamic and brainstem areas similar to those innervated by the CeAM that mediate the specific signs of fear and anxiety. The generality of this idea is illustrated by selective studies looking at context conditioning, social defeat, drug withdrawal and stress induced reinstatement.

Quantifying fear potentiated startle using absolute versus proportional increase scoring methods: implications for the neurocircuitry of fear and anxiety

Abstract Rationale. The fear-potentiated startle paradigm [increased startle in the presence of a conditioned fear stimulus (CS)] has become increasingly popular as a tool for evaluating the potential efficacy of putative anxiolytic compounds. However, when the tested compounds also influence baseline startle, it is unclear how comparisons with control groups can best be made. Objective. To evaluate the validity of absolute difference (startle amplitude on CS minus non-CS test trials) vs. proportional increase (the absolute difference score divided by startle amplitude on non-CS test trials) scoring methods. Methods. The effect on proportional increase and absolute difference scores of baseline shifts that occur with or without concomitant increases in fear was evaluated in rats. A reliable measure should yield similar scores across shifting baselines, provided that fear levels remain constant. Results. Preexisting baseline differences, and those brought about by different startle-eliciting noise burst intensities, by strychnine injections, or by CRH infusions, each increased absolute difference scores without markedly influencing proportional change scores. These baseline differences were not associated with different fear levels. Increases in baseline startle brought about by unsignaled footshocks or by a second CS – increases which are associated with increased fear – partially occluded additional CS-induced increases using either measure. Conclusions. Across different baselines, CS-elicited fear is most accurately reflected in proportional change scores. Under certain conditions saturation effects may interfere with an accurate assessment using either measure. However, these same saturation effects may provide opportunities to explore the neural circuitry of fear and anxiety in novel ways.

Lack of a temporal gradient of retrograde amnesia following NMDA-induced lesions of the basolateral amygdala assessed with the fear-potentiated startle paradigm.

M. Kim and M. Davis (1993b) previously reported that electrolytic lesions of the central nucleus of the amygdala, made 6 or 30 days after training, completely blocked the expression of fear-potentiated startle in rats. The present study shows that excitotoxic lesions of the basolateral amygdala also block fear-potentiated startle and do so whether the lesions are made soon (i.e., 6 days) or long (i.e., 30 days) after training. The relevance of these findings to various theories of amygdala function is discussed.

Opposing roles of the amygdala and dorsolateral periaqueductal gray in fear-potentiated startle.

The whole-body acoustic startle response is a short-latency reflex mediated by a relatively simple neural circuit in the lower brainstem and spinal cord. The amplitude of this reflex is markedly enhanced by moderate fear levels, and less effectively increased by higher fear levels. Extensive evidence indicates that the amygdala plays a key role in the potentiation of startle by moderate fear. More recent evidence suggests that the periaqueductal gray is involved in the loss of potentiated startle at higher levels of fear. The influence of both structures may be mediated by anatomical connections with the acoustic startle circuit, perhaps at the level of the nucleus reticularis pontis caudalis. The present chapter reviews these data.